docs(utils): add function signatures

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docs
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docs/utils.md
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# utility functions
## `_utils.mkVhost`
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`attrset -> attrset`
make virtual host with sensible defaults
pass in a set to override the defaults.
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### Example
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```nix
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services.nginx.virtualHosts."balls.example" = _utils.mkVhost {};
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```
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## `_utils.mkSimpleProxy`
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`attrset -> attrset`
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make a simple reverse proxy
takes a set: